1. Field of the Invention
The present invention concerns a method of operating an air-feed type spray booth and, more specifically, it relates to a method of operating an air-feed type spray booth, disposed to a coating line, having a tunnel-like booth through which articles to be spray-coated such as car bodies are successively conveyed and in which an air supplied from an air conditioner to a plenum chamber is forcively fed by a feed fan downwardly at a predetermined speed and then drawn together with painting mists, evaporated vapors of organic solvents, etc. entrained thereon down through the floor of the booth by an exhaust fan.
2. Description of the Prior Art
An air-feed type spray booth, for example, for applying spray coating on car bodies is usually disposed between a device for applying pre-treatment to the spray coating and a device for applying post-treatment such as a drying furnace in a coating line, along which the car bodies to be spray coated are successively passed on a conveyor. The spray booth has an elongate tunnel-like configuration in which a clean conditioned air supplied by a feed fan to the plenum chamber is forcively fed downwardly at a predetermined speed and then the air, after picking-up painting mists and vapors of organic solvents resulted from spray coating, is drawn downwardly through the floor of the booth by an exhaust fan, so as to suppress the scattering of the painting mists, dusts, etc. that would otherwise give undesired effects on the coated films thereby keeping the coating quality satisfactorily, as well as maintaining the working circumstance healthy where operators conduct preparation for the automatic coating or conduct spray coating manually in the spray booth.
In the air-feed type spray booth of the aforementioned structure, if the flow rate of the air fed from the air conditioner by the feed fan to the plenum chamber is different from the flow rate of the air drawn and exhausted down through the floor of the booth, external airs, for example, from the pre-treatment device or the drying furnace that contain dusts and the likes or are at an undesirably high temperature would intrude through the inlet or outlet opened at both ends of the booth to degrade the coating quality of the coated articles, or airs contaminated with the painting mists, solvent vapors, etc. are issued externally from the inside of the booth through the inlet or outlet to the pre-treatment device of the drying furnace to worsen the working conditions therein.
As the countermeasure for the foregoing disadvantages, the feed fan and the exhaust fan have heretofore been driven each at a predetermined constant number of rotation such that the flow rates are identical between the feeding air and the exhaust air thereby inhibiting the air streams from flowing inwardly and/or outwardly of the spray booth through the inlet and/or outlet thereof to the devices at the upstream and downstream.
However, in the air-feed type spray booth usually adapted to draw and exhaust the air through a plurality of sucking ports formed as the slits in the floor of the booth to the beneath of the floor, spray coated articles such as car bodies are successively conveyed on the floor of the booth, while closing or exposing the sucking ports as they move continuously through the booth. Accordingly, the open area of the sucking ports and thus the flow rate of the exhaust escaping through the ports vary depending on the number of the interval of the car bodies conveyed on the floor of the booth.
Then, even if the number of rotation for the feed fan and the exhaust fan is set to a certain level as in the prior art, the balance between the flow rates of the feed and exhaust airs may be lost depending on the manner that the car bodies, etc. are conveyed through the spray booth.
Further, below the floor of the spray booth, there is disposed a mist treating chamber comprising a venturi device for separating to remove the painting mists by the gas-liquid contact of the drawn exhaust air with water therein and a water tank for recoverying water supplied to the venturi device.
Then, if the amount of water supplied to the venturi device or the amount of water stored in the water tank changes, the working load imposed on the exhaust fan varies to possibly fluctuate the number of rotation thereof and break the balance between the feed air and the exhaust air. Imbalance between the flow rate of the feed air and that of the exhaust air leads to various disadvantages as described above. Specifically, if the flow rate of the exhaust air is predominant, liquid chemicals such as a processing liquid for chemical formation for the pre-treatment device upstream to the spray booth or a hot air stream at a high temperature of about 150.degree.-200.degree. C. from the drying furnace downstream to the spray booth may be flown to the inside of the booth, thereby deteriorating the working circumstance in the spray booth or degrading the coating quality. On the other hand, if the feed air is predominant, it causes the air in the spray booth usually conditioned to about 25.degree. C. to be released in a great amount and flow into the downstream drying furnace, which may possibly lower the temperature in the furnace suddenly, thereby, result in defective baking, etc.
However, there have been known no effective countermeasure for such disadvantages of the air-feed type spray booth in the prior art.